This invention relates to a process for the preadjustment and adjustment of the change-over point of a brake pressure control unit of a pressure medium-actuatable vehicle brake unit. The brake pressure control unit is load-sensitively controlled by an adjustable spring device and is equipped with at least one valve with a control piston being pressure-sensitively slidable into closing position against the force of the spring device pivotable to a swiveling transmission lever. The transmission lever is moved between two defined positions, one of which being defined by the closing position of the valve. The present invention is more particularly, related to a brake pressure control unit for a pressure medium-actuatable vehicle brake unit, which brake pressure control unit is preadjustable and adjustable by means of the process and is load-sensitively controllable by an adjustable spring device and which is equipped with at least one valve with a control piston being pressure-sensitively slidable into closing position against the force of the spring device pivotable to a swiveling transmission lever.
In load-sensitive brake pressure control units, the change-over point, that is the pressure valve at which the pressure control initiates, is variable depending on the load of the vehicle. For this purpose, a swiveling transmission lever is arranged at the brake pressure control unit against which transmission lever the control piston of the valve of the brake pressure control unit is abutted and to which one end of a spring device is pivoted whose other end is pivoted to a vehicle element whose position relative to the brake pressure control unit is variable depending on the load. When the vehicle load varies, the force of the spring device acting upon the transmission lever will vary accordingly.
It is difficult and labor-intensive to mount the brake pressure control unit in the vehicle while under a defined load and to connect the spring device with the vehicle element and to adjust it in such a manner that the real change-over pressure coincides with the value desired in consideration of the brake pressure characteristics.
A process for the preadjustment and adjustment of the change-over point of a brake pressure control unit is known from the German patent application No. 2,945,027 by which a preadjustment is made prior to mounting of the brake pressure control unit in the vehicle and according to which the adjustment is made in the course of mounting of the brake pressure control unit in the vehicle. In the prior art process, first the spring pivotal point of the transmission lever of the unmounted brake pressure control unit is adjusted into a spatially exactly defined first position in that a first adjustment point provided at the casing of the brake pressure control unit is brought into a defined distance relative to a second adjustment point arranged at the transmission lever by means of a distance gauge. In this defined position, the value is adjusted in such a manner that the valve closing organ just comes to be in contact at the pertaining valve seat, that is, that it closes, which is ascertained by a flow measurement. Subsequently, during mounting of the brake pressure control unit in the definedly loaded vehicle, the spring pivotal point is brought into an exactly defined second position. In doing so, the distance between the two positions of the spring pivotal point is selected such that it exactly corresponds to the spring excursion by which the spring device requires to be extended to reach the closing position, in order to furnish the amount of spring force necessary to adjust the desired change-over point. This second defined position, too, is adjusted by means of a second distance gauge and is thus fixed. Upon adjustment and fixation of the second position, the spring device is suspended free of play and force at the transmission lever and at a vehicle element, and the fixation of the second position is cancelled. The application of this process for the preadjustment and adjustment of the change-over point is based on the assumption that a spring device which has to sustain stretching strains requires to be elongated by a defined spring excursion in order to bring to bear an exactly defined spring force, in other words, the adjustment of the predetermined spring force is effected by way of adjustment of the spring excursion determined on the basis of the theoretical spring parameters. It is a disadvantage in this context and inherent in this process that the strong deviations occurring in practice in the characteristic data of the individual springs cannot be taken account of. If and when precision springs with narrow tolerances are employed, this will result in an increase of the costs of the whole brake pressure control device. In addition, two different gauges are required to carry out the process. Since the change-over pressure and the load-sensitive variation of the center distance are generally determined by the constructional design, there result undesirably extended valve closing travels which may lead to an excessive wear of the slidable components.
It is an object of the present invention to create a specific process for the preadjustment and adjustment of the change-over point of a brake pressure control unit which is easy to carry out and in which the influence the spring tolerances bear on the precision of adjustment of the change-over point is excluded. A further object of the present invention is to create a brake pressure control unit of the kind mentioned in the beginning which is suited for the application of the inventive process and whose change-over point allows to be preadjusted in the manufacturers' works and adjusted in the event of mounting in the vehicle without cumbersome adjusting operations.